1. Field of the Invention
This invention relates to a nitride III-V compound semiconductor substrate, its manufacturing method, a method of manufacturing a semiconductor light emitting device and a method of manufacturing a semiconductor device, which are especially suitable for use in manufacturing semiconductor lasers and light emitting diodes, or electron transporting devices.
2. Description of the Related Art
In recent years, semiconductor lasers using nitride III-V compound semiconductors such as AlGaInN (hereinbelow called GaN compound semiconductor lasers) have been under active research and development in the hope of making semiconductor lasers capable of emitting light over the range from the blue region to the ultraviolet region necessary for enhancing the density of optical discs. Lately, efforts are being expended to farther improve their lifetimes and properties toward their practical use.
When manufacturing such a GaN compound semiconductor laser, a laser structure is most typically formed by crystal growth of a GaN compound semiconductor layer on a sapphire substrate. For example, using a sapphire substrate sized 50 mm (2 inches) in diameter and 430 xcexcm in thickness, a GaN compound semiconductor layer is grown thereon up to a thickness around 7 xcexcm in total.
However, if a GaN compound semiconductor has a thickness around 7 xcexcm on a sapphire substrate as mentioned above, the sapphire substrate warps due to a difference in thermal expansion coefficient between the sapphire and the nitride III-V compound semiconductor such as GaN. This warpage measures as large as 80 xcexcm.
This large warpage of the sapphire substrate adversely works against exposure in a manufacturing process of a GaN compound semiconductor laser and polishing of the bottom surface of the sapphire substrate.
More specifically, in the exposure process, the sapphire substrate, having GaN compound semiconductor layers grown thereon and a resist coated on its surface, undergoes exposure through a photo mask. If the sapphire substrate largely warps as mentioned above, distance between the photo mask and the resist may become uneven within the area of the substrate, or a dimensional deviation may be produced between the photo mask and the substrate within the area of the substrate. Thus the mask cannot accurately fit the entire surface of the substrate. As a result, especially when a base GaN layer is laterally grown on the sapphire substrate by ELO (epitaxial lateral overgrowth) and GaN compound semiconductor layers forming a laser structure are grown thereon by crystal growth, it is difficult to form a ridge in a less-defective region (wing portion) between a seed crystal and a coalescing portion of the lateral growth, and the ridge often deviates from the wing portion. Therefore, this problem adversely affects the laser properties, and in particular, its lifetime, and also degrades the production yield.
For making cavity edges, it is the most usual way to cleave a sapphire substrate having GaN compound semiconductor layers grown thereon. For easier cleavage, it is necessary to thin the sapphire substrate by partly removing it from the bottom by polishing. However, if the sapphire substrate largely warps as mentioned above, it often cracks during polishing.
Furthermore, if the warpage of the sapphire substrate is large during crystal growth of the GaN compound semiconductor layers, because of uneven temperature distribution along the plane, the resulting GaN compound semiconductor layers become uneven in composition and thickness.
Under the circumstances, there is a demand for a technique capable of sufficiently reducing warpage of substrates, namely, not to exceed 70 xcexcm, to remove the above-mentioned problems caused by warpage of sapphire substrates.
It is therefore an object of the invention to provide a nitride III-V compound semiconductor substrate and its manufacturing method capable of limiting warpage of substrates not to exceed 70 xcexcm.
A further object of the invention is to provide a method of manufacturing a semiconductor light emitting device that can be used when manufacturing the semiconductor light emitting device by using a nitride III-V compound semiconductor substrate made by forming a nitride III-V compound semiconductor layer on a substrate of a material different from the nitride III-V compound semiconductor layer, and can limit warpage of the substrate not to exceed 70 xcexcm, thereby to successfully carry out exposure in the lithographic process and polishing of the bottom surface of the substrate.
A still further object of the invention is to provide a method of manufacturing a semiconductor device that can be used when manufacturing the semiconductor device by using a nitride III-V compound semiconductor substrate made by forming a nitride III-V compound semiconductor layer on a substrate of a material different from the nitride III-V compound semiconductor layer, and can limit warpage of the substrate not to exceed 70 xcexcm, thereby to successfully carry out exposure in the lithographic process and polishing of the bottom surface of the substrate.
The Inventor conducted vigorous research to solve the above-indicated problems. An outline of the research is shown below.
GaN layers were grown on sapphire substrates sized 430 xcexcm and 700 xcexcm in thickness, and 50 mm in diameter. FIG. 1 shows measured values of thickness of GaN layers and measured values of warpage (H) of the sapphire substrates. For growth of the GaN layers, metal organic chemical vapor deposition (MOCVD) was used. It is appreciated from FIG. 1 that the warpage of the sapphire substrate increases proportionally to the thickness of the GaN layer.
In case a laser structure of GaN compound semiconductor layers is formed on the sapphire substrate, for the purpose of forming a less-defective layer by using a lateral growth technique such as ELO or preventing the operation voltage from increasing when both the n-side electrode and the p-side electrode are formed on a common plane, thickness of the n-type GaN layer grown as the base layer of the laser structure on the sapphire substrate is preferably not smaller than 3 to 5 xcexcm. However, if the GaN layer is grown by 5 xcexcm on the sapphire substrate sized 430 xcexcm in thickness and 50 mm in diameter, the warpage exceeds 70 xcexcm as shown in FIG. 1. In contrast, if the GaN layer is grown by 5 xcexcm on the sapphire substrate sized 700 xcexcm in thickness and 50 mm in diameter, the warpage largely decreases to around 30 xcexcm. As such, when GaN compound semiconductor layers forming a laser structure (cladding layer, waveguide layer, active layer, and so on, which are approximately 2 xcexcm thick in total) are formed on a 5 xcexcm thick base GaN layer, the increase of warpage of the substrate is small. That is, warpage largely depends on the thickness of the base GaN layer.
FIG. 2 shows measured values of warpage of diametrically 50 mm long and x(xcexcm) thick sapphire substrates having y(xcexcm) thick GaN layers thereon, putting Z=y/x on the abscissa and warpage H(xcexcm) on the ordinate. It is appreciated from FIG. 2 that the warpage reaches and surpasses 80 xcexcm under Z in excess of 0.013, and it adversely affects the mask-fitting in the photolithographic process and polishing of the substrate bottom surface. If y/x is adjusted not to exceed 0.011, the warpage can be limited not to exceed 70 xcexcm. To ensure that the warpage does not exceed 40 xcexcm to enhance the production yield, Z must be 0.006 or less.
Basic data of FIGS. 2 and 3 is collectively shown in FIG. 3.
Also from the viewpoint of improving evenness of the temperature distribution along the substrate plane during crystal growth, warpage of the substrate must be minimized. Evenness of the temperature distribution is required for ensuring evenness of the Al composition distribution of the cladding layer of the GaN compound semiconductor laser, distribution of the thickness of each layer and distribution of composition of the active layer (corresponding to the distribution of the emission wavelength). Especially, the active layer needs accurate temperature control and temperature distribution control because the emission wavelength changes by approximately 1 nm with change of the growth temperature by 1xc2x0 C. If warpage occurs in the substrate, the temperature distribution on the substrate surface increases, and the composition distribution of the active layer (distribution of the emission wavelength) also increases. This results in increasing the region where the emission wavelength deviates from the desired wavelength and decreasing the production yield. If Z is decreased to 0.011 or less and the warpage is limited not to exceed 70 xcexcm, then the production yield can be enhanced also from the above-mentioned viewpoint.
On the other hand, warpage of the substrate depends on the substrate size as well. Even when Z is large, if the substrate is small, warpage can be reduced. Taking this into consideration, the relation between Z and the substrate diameter D for limiting the warpage H below a certain value was found as Equation (1) below and as shown in FIG. 4.
0 less than D less than (2/CZ)cosxe2x88x921(1xe2x88x92HCZ)xe2x80x83xe2x80x83(1)
where Z=y/x and C are constants determined by the thickness of the substrate.
Equation (1) is obtained as follows. Assume here that a substrate having the diameter D (cm) warps as shown in FIG. 5. If the warpage is H (cm), the radius of curvature of the substrate is xcfx81 (cmxe2x88x921), and the view angle of the full span of the substrate from the curvature center is xcex8 (rad), then they have the following relations.
xcfx81xc2x7xcex8=Dxe2x80x83xe2x80x83(2)
H=xcfx81(1xe2x88x92cos(xcex8/2))xe2x80x83xe2x80x83(3)
An experiment made by the Inventor demonstrates that the curvature 1/xcfx81 is proportional to Z=(thickness of nitride III-V compound semiconductor layer)/(thickness of substrate)=y/x, in the following relationship.
1/xcfx81=CZxe2x80x83xe2x80x83(4)
where C (cmxe2x88x921) is a proportionality constant that is definitely determined by a fixed thickness of the substrate of a particular material.
As an example, in case the substrate is a 430 xcexcm thick sapphire substrate and the overlying nitride III-V compound semiconductor layer is a GaN layer, Z and 1/xcfx81 make the relation shown in FIG. 6. At the time the proportionality constant C is C=0.20567 (cmxe2x88x921).
From Equations (2), (3) and (4), the relation between D and Z is as follows.
D=(2/CZ)cosxe2x88x921(1-HCZ)xe2x80x83xe2x80x83(5)
By determining the substrate diameter to be smaller than D obtained by Equation (5) for the warpage H given, the warpage of the substrate can be limited not to exceed H. That is, when this substrate diameter is D and taking D greater than 0 into account, by determining the diameter D of the substrate to satisfy
0 less than D less than (2/CZ)cosxe2x88x921(1-HCZ)xe2x80x83xe2x80x83(6)
the warpage of the substrate can be limited not to exceed H. The condition for limiting the warpage of the substrate not to exceed 70 xcexcm is 0 less than Hxe2x89xa670xc3x9710xe2x88x924 (cm), and the condition for limiting the warpage of the substrate not to exceed 40 xcexcm is 0 less than Hxe2x89xa640xc3x9710xe2x88x924 (cm).
Although the foregoing explanation was directed to circular substrates, the invention is not limited to circular substrates, but it is applicable to other shapes of substrates within the limit not inviting difficulties from the viewpoint of the process and the like. In this case, the maximum dimension along the substrate plane may be regarded as D.
Further, the foregoing explanation is also applicable to substrates other than sapphire substrates provided those other substrates have a difference in thermal expansion coefficient from that of the nitride III-V compound semiconductor, which is equivalent to or lower than the difference the sapphire substrates have.
The invention has been made through further research based on the above-explained research by the Inventor.
According to the first aspect of the invention, there is provided a nitride III-V compound semiconductor substrate in which nitride III-V compound semiconductor layers are formed on a substrate of a material different from those of the nitride III-V compound semiconductor layers, comprising:
0 less than y/xxe2x89xa60.011 and xxe2x89xa7450 xcexcm
being satisfied when thickness of the substrate is x (xcexcm) and thickness of the nitride III-V compound semiconductor layers is y(xcexcm).
According to the second aspect of the invention, there is provided a method of manufacturing a nitride III-V compound semiconductor substrate in which nitride III-V compound semiconductor layers are formed on a substrate of a material different from those of the nitride III-V compound semiconductor layers, comprising:
0 less than y/xxe2x89xa60.011 and xxe2x89xa7450 xcexcm
being satisfied when thickness of the substrate is x (xcexcm) and thickness of the nitride III-V compound semiconductor layers is y (xcexcm).
According to the third aspect of the invention, there is provided a method of manufacturing a semiconductor light emitting device using a nitride III-V compound semiconductor substrate in which nitride III-V compound semiconductor layers are formed on a substrate of a material different from those of the nitride III-V compound semiconductor layers, comprising:
just before the start of at least one process, the nitride III-V compound semiconductor substrate satisfying
0 less than y/xxe2x89xa60.011 and xxe2x89xa7450 xcexcm
when thickness of the substrate is x (xcexcm) and thickness of the nitride III-V compound semiconductor layers is y (xcexcm).
According to the fourth aspect of the invention, there is provided a method of manufacturing a semiconductor device using a nitride III-V compound semiconductor substrate in which nitride III-V compound semiconductor layers are formed on a substrate of a material different from those of the nitride III-V compound semiconductor layers, comprising:
just before the start of at least one process, the nitride III-V compound semiconductor substrate satisfying
xe2x80x830 less than y/xxe2x89xa60.011 and xxe2x89xa7450 xcexcm
when thickness of the substrate is x (xcexcm) and thickness of the nitride III-V compound semiconductor layers is y (xcexcm).
In the first to fourth aspects of the invention, the relation 0 less than y/xxe2x89xa60.006 is preferably satisfied to limit the warpage of the substrate not to exceed 40 xcexcm.
According to the fifth aspect of the invention, there is provided a nitride III-V compound semiconductor substrate in which nitride III-V compound semiconductor layers are formed on a substrate of a material different from those of the nitride III-V compound semiconductor layers, comprising:
0 less than D less than (2/CZ)cosxe2x88x921(1xe2x88x92HCZ)
being satisfied when the maximum dimension of the substrate is D (cm), warpage of the substrate is in the range of 0 less than Hxe2x89xa670xc3x9710xe2x88x924 (cm), Z=(thickness of nitride III-V compound semiconductor layers)/(thickness of the substrate), and C (cmxe2x88x921) is the proportionality constant when the radius of curvature of the substrate xcfx81 (cm) is expressed as 1/xcfx81=CZ.
According to the sixth aspect of the invention, there is provided a method of manufacturing a nitride III-V compound semiconductor substrate in which nitride III-V compound semiconductor layers are formed on a substrate of a material different from those of the nitride III-V compound semiconductor layers, comprising:
0 less than D less than (2/CZ)cosxe2x88x921(1-HCZ)
being satisfied when the maximum dimension of the substrate is D (cm), warpage of the substrate is in the range of 0 less than Hxe2x89xa670xc3x9710xe2x88x924 (cm), Z=(thickness of nitride III-V compound semiconductor layers)/(thickness of the substrate), and C (cmxe2x88x921) is the proportionality constant when the radius of curvature of the substrate xcfx81 (cm) is expressed as 1/xcfx81=CZ.
According to the seventh aspect of the invention, there is provided a method of manufacturing a semiconductor light emitting device using a nitride III-V compound semiconductor substrate in which nitride III-V compound semiconductor layers are formed on a substrate of a material different from those of the nitride III-V compound semiconductor layers, comprising:
just before the start of at least one process, the relation
0 less than D less than (2/CZ)cosxe2x88x921(1-HCZ)
being satisfied when the maximum dimension of the substrate is D (cm), warpage of the substrate is in the range of 0 less than Hxe2x89xa670xc3x9710xe2x88x924 (cm), Z=(thickness of nitride III-V compound semiconductor layers)/(thickness of the substrate), and C (cmxe2x88x921) is the proportionality constant when the radius of curvature of the substrate xcfx81 (cm) is expressed as 1/xcfx81=CZ.
According to the eighth aspect of the invention, there is provided a method of manufacturing a semiconductor device using a nitride III-V compound semiconductor substrate in which nitride III-V compound semiconductor layers are formed on a substrate of a material different from those of the nitride III-V compound semiconductor layers, comprising:
just before the start of at least one process, the relation
0 less than D less than (2/CZ)cosxe2x88x921(1xe2x88x92HCZ)
being satisfied when the maximum dimension of the substrate is D (cm), warpage of the substrate is in the range of 0 less than Hxe2x89xa670xc3x9710xe2x88x924 (cm), Z=(thickness of nitride III-V compound semiconductor layers)/(thickness of the substrate), and C (cmxe2x88x921) is the proportionality constant when the radius of curvature of the substrate xcfx81 (cm) is expressed as 1/xcfx81=CZ.
In the fifth to eighth aspects of the invention, the warpage H preferably satisfies 0 less than Hxe2x89xa640xc3x9710xe2x88x924 (cm).
In the third, fourth, seventh and eighth aspects of the invention, the warpage of the substrate is preferably limited not to exceed 70 xcexcm, more preferably no to exceed 40 xcexcm, in a process largely affected by warpage of the substrate, such as a photolithographic process especially during its exposure, or just before starting a bottom polishing process.
In the present invention, the smaller the warpage of the substrate, the easier the process. However, excessive effort to reduce the warpage will result in excessively increasing the substrate thickness or excessively reducing the substrate diameter, and will degrade the productivity. On the other hand, although there is no essential upper limit of the substrate thickness, if the substrate is excessively thick, it takes large amounts of time for polishing of the substrate bottom and dicing, and it results in increasing the cost of the substrate itself. From the practical viewpoint, if the substrate is preferably thinner than 1 mm (1000 xcexcm), it is effective to shorten the time required for polishing of the substrate bottom or dicing and to limit the cost of the substrate itself. In case the nitride III-V compound semiconductor to be formed on the substrate is desired to be thicker, thickness of the substrate must be increased unless the substrate size is reduced. Taking these factors into consideration, thickness of the substrate is preferably determined in the range of 600 xcexcmxe2x89xa6x less than 1000 xcexcm or more preferably in the range of 700 xcexcmxe2x89xa6x less than 1000 xcexcm. If the substrate is excessively small, each substrate yields fewer semiconductor light emitting devices or semiconductor devices, and the productivity degrades. Therefore, the substrate is preferably sized at least to have an area equivalent to that of a 1-inch substrate in diameter.
In the present invention, the nitride III-V compound semiconductor is composed of at least one kind of group III elements selected from the group consisting of Ga, Al, In and B, and one or more group V elements including at least N and may further include As and/or P. Examples of the nitride III-V compound semiconductor are GaN, AlGaN, AlN, GaInN, AlGaInN, InN, and the like.
The substrate for growing the nitride III-V compound semiconductor layer can be chosen from various kinds of substrates. For example, a sapphire substrate, SiC substrate, Si substrate, GaAs substrate, GaP substrate, InP substrate, spinel (MgAl2O4) substrate or silicon oxide substrate may be used.
For growth of the nitride III-V compound semiconductor, any appropriate technique such as metal organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxial growth or halide vapor phase epitaxial growth HVPE), for example, may be used.
The semiconductor light emitting device can be a semiconductor laser or a light emitting diode, for example. Basically, the semiconductor device may be of any type that use the nitride III-V compound semiconductor. For example, it may be a light emitting device such as a semiconductor laser or light emitting diode, or an electron transporting device such as FET or heterojunction bipolar transistor.